High pressure valves for controlling flow are important devices in the production of oil and gas. In the trade such valves are commonly referred to as adjustable chokes. A positive choke is a fixed size orifice placed in the flow line at a wellhead, for example, to limit flow of fluid from the well. It is sometimes desirable to employ an adjustable choke so that production from the well can be controlled over a range of flow rates. Such an adjustable choke or valve is placed in the "Christmas tree" at the wellhead and set to a selected size opening corresponding to a selected size positive choke for controlling production of oil or gas from the well.
The valve may sit untended for long periods of time with prolonged exposure to corrosive environments and in many instances to extremely erosive conditions where high flow velocities and/or particles are present in the fluid flowing through the valve. When such a choke is closed, it should seat tightly so that no leakage occurs. For this reason erosion of the valve seat can be a severe problem.
It is also desirable to localize erosion in a portion of the valve that is easily replaced in the field. The flow control structure is preferably readily removable so that a different size range of choke can be substituted without removing the valve from the Christmas tree assembly. Safety is an important consideration during such maintenance of a valve. The valves are commonly used in very high pressure systems and unless the pressure inside the valve is relieved before disassembly, attempts to remove the bonnet of the valve could result in severe injury to workmen.
Thus, it is highly desirable to provide a high pressure choke giving a broad range of flow control with minimized erosion on the valve seat, safety, and ease of maintenance.